Polyvinyl chloride (PVC) is one of the largest volume commodity thermoplastics manufactured industrially. More than 75% of PVC is made using suspension polymerisation technology. The final physical form or morphology of the polymer is determined by a combination of factors in the polymerisation process. Two characteristics of the product which are particularly important are the particle size and the porosity of the particles.
If the particle size is too low or too high, subsequent processing of the polymer to form molded or extruded articles is made more difficult. It is therefore particularly desirable for most applications to produce PVC particles with a mean grain size of the order of 100 to 200 μm, and a narrow particle size distribution such that no more than a small percentage of particles are smaller than 63 μm or larger than 250 μm.
High porosity is important, because it facilitates the removal of unreacted VCM from the PVC particles. VCM is a known carcinogen, and the industry is therefore continually attempting to reduce the level of monomer to well below 1 ppm in the final resin. In the case of PVC-based foils for medical applications, residual monomer levels less than 50 ppb are required. High porosity means that unreacted VCM can be removed efficiently from the particles at lower temperatures, thus avoiding problems of thermal degradation of the PVC at higher temperatures. High porosity also facilitates the subsequent introduction of processing additives (such as plasticisers) into the particles.
To a large extent, the final particle size and the amount of porosity in the polymer is determined by the polymerisation temperature, the prevailing agitation conditions in the polymerisation reactor, and by the use of certain additives which are variously referred to as “dispersants” or “protective colloids”.
Protective colloids are categorised as either primary or secondary types. Primaries are largely responsible for the control of the size of the polymer particles and secondaries are largely responsible for controlling the porosity of the polymer particles. Primary protective colloids are typically water soluble polymers based on substituted cellulose ethers and/or high molecular weight partially hydrolysed polyvinyl acetates (PVA). Commercially used secondary protective colloids are predominantly based on lower molecular weight partially hydrolysed PVA, and to a lesser extent surfactant technology.
The PVC industry is continually seeking to improve the suspension polymerisation process with the aim of improving polymer quality and improving the economics of production. The present invention overcomes some of the limitations of the existing commercially available secondary protective colloid technology.
The partially hydrolysed PVAs which are used in the majority of industrial applications as secondary protective colloids are manufactured in a two stage process. In the first stage, vinyl acetate is polymerized to PVA. In the second stage, the PVA is partially hydrolysed to form a polyvinyl acetate—polyvinyl alcohol copolymer. The particular polymerization conditions used in the first stage have a profound effect on the properties of the final product, including higher order polymer characteristics such as the level of unsaturation, conjugated unsaturation, chain branching and the nature of end groups. The properties of the final product are also dependent on the hydrolysis method. For example, base hydrolysis leads to products having a “blocky” structure, i.e. polymers having sequences of hydroxyl groups interspersed among sequences of pendent acetate groups. Acid hydrolysis, on the other hand, leads to randomly hydrolysed polymers.
As a result of the different manufacturing methods used, commercially available PVA protective colloid products which are reported to have nominally the same physical and chemical properties (measured by typical standard quality control methods, such as the degree of hydrolysis, solution viscosity etc.), can actually behave very differently in the suspension polymerisation process. This, in turn, leads to undesired variability in the resulting PVC product. It is therefore an aim of the present invention to provide secondary protective colloids which can be manufactured by a simplified process, resulting in reduced variability in its properties.
According to EP-A-0446747, the preferred acetate distribution for improved PVC internal structural homogeneity is found using PVA based protective colloids with a block like structure. However these products have a relatively small share of the secondary protective colloids market as they tend to exert a very strong primary effect in the suspension polymerization process, resulting in unacceptably low particle size PVC. It is therefore a further aim of the present invention to provide the improved PVC internal structural homogeneity characteristic of blocky PVAs, while maintaining a sufficiently high average particle size to produce commercial quality PVC.
The efficiency with which a secondary protective colloid introduces porosity into the PVC is significantly affected by the temperature at which the polymerisation is carried out. In practice, the porosity of the polymer is inversely proportional to the polymerisation temperature when all other polymerisation variables are kept constant. Low K value polymers prepared at high polymerisation temperatures generally have low porosity. It is therefore a still further aim of the present invention to provide secondary protective colloids which can produce high porosity at high polymerisation temperatures.
A major concern for PVC manufacturers is the quality of the PVC's initial colour once it has been formulated into a melt state. A number of process variables are known to influence the initial colour, including the choice of additives used during the suspension polymerization process. Intense research efforts have been focused on understanding the factors which contribute to PVC thermal stability, with little regard to the inherent thermal stability of the protective colloid. In the industrial manufacture of PVA based protective colloids, great care must be taken to avoid burning the PVA particularly during the stripping stage to remove residual vinyl acetate monomer. Poor vinyl acetate stripping can result in dark or black contamination in the final protective colloid product. A further aim of this invention is therefore to provide secondary protective colloids with improved thermal stability compared to current technology.
Acrylic based polymers have been developed which act as primary protective colloids in the suspension polymerisation of vinyl chloride. U.S. Pat. No. 4,104,457 describes acrylic acid copolymers to control particle size in bulk vinyl chloride polymerisations. U.S. Pat. No. 4,603,151 and U.S. Pat. No. 4,684,668 describe the use of cross-linked acrylic acid primary protective colloids which act as thickening agents. WO97/08212 describes the use of high molecular weight (>100000) acrylic dispersants which have a cloud point above the polymerisation temperature. These polymers are stated to act as primary colloids when cross-linked, and as secondary protective colloids when not cross-linked. The use of high molecular weight acrylic acid copolymers generally requires the addition of a base to the suspension polymerisation process to neutralise the acrylic acid. It is therefore an aim of the present invention to provide protective colloids which do not require neutralisation in the suspension polymerisation process.
EP-A-0483051, U.S. Pat. No. 5,155,189 and U.S. Pat. No. 5,244,995 describe the use of relatively low molecular weight polymers in suspension polymerization processes. The claimed additives are based on homo or copolymers containing more than 50% by weight of α,β-unsaturated ester(s) of acrylic acid and/or methacrylic acid having no ionic side groups. It is stated that terminal functional groups can be added to improve performance. These materials can be shown to act as secondary protective colloids increasing the porosity of the final PVC when compared to PVC produced without a secondary protective colloid. However, as described in WO97/08212, the use of low molecular weight polyacrylate colloids requires the use of a primary colloid, PVA, having a comparatively low (e.g. 72.5%) degree of hydrolysis. It is known that primary colloids of this type tend to increase porosity when compared to higher molecular weight/higher hydrolysis primary colloids. The application of acrylic secondary protective colloids of the type described in EP-A-0483051 is therefore of limited industrial value, as it restricts the choice of primary colloid to low molecular weight primaries. Industrially it is desirable to have a free choice of primary colloid to include higher hydrolysis primaries, non PVA-based primaries and mixtures of primaries in order optimise the polymerisation process. It is therefore an additional aim of the present invention to provide secondary protective colloids which can be used with a wide range of primary colloid types.
U.S. Pat. No. 4,579,923 describes the use of a monomeric hydroxy alkyl acrylate/propylene oxide adduct added at the start of PVC polymerisation to provide steric stabilisation of primary particles, hence producing porosity. Industrially, VCM polymerisations are not allowed to proceed to 100% conversion as the thermal stability of the resulting PVC is adversely affected at very high conversion. Therefore some unreacted VCM is always recycled back into the suspension polymerisation process. If a second unreacted monomer is present in the recycled VCM, expensive equipment to purify the monomer may be necessary to ensure a consistent high quality PVC product. Moreover, it is known that the addition of even small amounts of free monomers into the suspension polymerisation process can significantly retard the rate of polymerisation and although this can be compensated by the use of more initiator, economically it is not favoured. It is therefore a still further aim of the present invention to provide secondary protective colloids which have little or no effect on the polymerisation kinetics, and which do not represent a contaminant in the recycled VCM.
Suspension PVC production is a batch process and major developments in the last few years have focussed on improving the productivity of the polymerisation reactors. The process of making PVC can be split into the actual time polymerising VCM, referred to as the reaction time, and the time taken to charge and discharge the reactor, referred to as the non-reaction time. One way in which the non-reaction time can be reduced is to charge the reactor with hot water to reduce the non-reaction time. It is therefore an additional aim of the present invention to provide secondary protective colloids which can be used in hot water charge processes.
It is now common practice for large PVC reactors to remove significant amounts of heat of polymerisation via a reflux condenser. This increases the efficiency of the reaction and therefore reduces reaction time. However, as US patent application US 2003/0162927 states, the time at which the condenser is effectively turned on can have a detrimental effect on the quality of the PVC, generally increasing the particle size distribution to an unacceptable level. It is therefore a still further aim of the present invention to provide secondary protective colloids which provide improved particle size stability early on in the polymerization, thus allowing higher rates of heat removal via a reflux condenser, even from the start of reaction.